Method for the heat treatment of solids

ABSTRACT

In a process for the thermal treatment of solid materials ( 3 ), in particular refuse, in which the solid materials ( 3 ) are burnt/gasified or pyrolized in a first step ( 5 ) with a lack of oxygen, and then, in an afterburning zone ( 14 ), the flue gases ( 6 ) from the first step ( 5 ) are mixed with an oxygen-containing gaseous medium ( 15 ) and are burnt with complete burn-off, the flue gases ( 6 ) emerging from the first step ( 5 ) are firstly actively homogenized in a mixing zone ( 7 ) with the addition of a gaseous oxygen-free or low-oxygen medium ( 8 ) before they are mixed with the oxygen-containing medium ( 15 ). Then, the homogenized flue-gas stream flows through a holding zone ( 13 ), in which it stays for at least 0.5 second, before, in an afterburning zone ( 14 ), the medium ( 15 ) which serves to ensure complete burn-off of the flue gas is added. The process according to the invention is distinguished by simple process steps and by a reduced level of pollutant emissions, in particular NOx, compared to the prior art.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for the thermal treatment of solidmaterials, in particular refuse, such as domestic and community waste,in which the solid materials are burnt/gasified or pyrolized in a firststep with a lack of oxygen, and then, in an afterburning zone, the fluegases from the first step are mixed with an oxygen-containing gaseousmedium and are burnt with complete burn-off.

2. Background of the Invention

It is known in the prior art to burn lumpy solid materials, such as forexample refuse, in a combustion chamber to which primary air is added,and a downstream afterburning chamber, to which secondary air is added.Usually, in this case, the solid material is moved on a combustiongrate. The primary air is fed in beneath the grate and flown throughopenings in the grate covering into the bed of solid material lyingabove the grate.

The flue gases which are formed in and above the bed during combustionhave a composition and temperature which fluctuate considerably locallyand over the course of time. Therefore, in conventional systems, theseflue gases are subsequently mixed with the aid of secondary air orsecondary air and recirculated flue gas. The secondary air fulfills thefollowing functions:

mixing the gases emerging from the combustion chamber

supplying oxygen in order to ensure burn-off of the gases

cooling of the emerging gases.

The primary air added in the first step is usually sufficient tocompletely burn the fuel, and the secondary air is used to achievecross-mixing of the flue gas (mixing of CO-containing gas trains withO₂₋containing gas trains). To ensure sufficient mixing, the amount ofsecondary air blown in must be selected to be suitably high. However,this excess air has the drawback of increasing the volume of flue gas.

In order to eliminate this drawback, EP 0,607,210 B1 describes a processfor the combustion of solid materials, in which apart from the primaryair no further combustion air is fed into the combustion boiler. Toimprove the poor burn-off of the gases which is caused by insufficientmixing in the afterburning chamber and which leads to high pollutantlevels in the flue gas, it is proposed in EP 0,607,210 B1, on the onehand to add sufficient primary air to provide an excess of oxygen asearly as in the first step, and on the other hand to inject water steaminto the combustion boiler above the combustion space and in the lowerarea of the afterburning chamber at an ultrasonic speed produced byexcess pressure. This process has the drawback that, in the event ofthere being an excess of air in the first combustion step, much of thenitrogen contained in the fuel is oxidized to form NO, and consequentlyit is impossible to achieve low NOx emissions.

A further process for the thermal treatment of refuse is known(Beckmann, M. and R. Scholz: “Vergasung von Abfällen” [Gasification ofRefuse] in “Vergasungsverfahren für die Entsorgung von Abfällen”[Gasification Process for Disposing of Refuse], Springer-VDI-VerlagGmbH, Düsseldorf, 1998, pp. 80-109), in which process the volume ofprimary air beneath the grate is reduced to such an extent that the fuelis gasified and a CO-rich flue gas is formed. In a following, completelyseparate afterburning chamber, this flue gas is afterburnt with air.Although the considerable reduction in the addition of air in the firststep is reported to provide an advantageous clear reduction in the NOxemissions compared to conventional grate combustion systems, hithertothis process has only been carried out on trial scale. The afterburningchamber was completely separate from the combustion chamber andconnected by a pipe. The flue-gas stream was homogenized by means ofturbulence when it flowed through this pipe. As a result of the smallbatch size and of the flue-gas stream being guided out of the primarycombustion chamber through a connection pipe, it was possible todispense with a device for mixing the flue-gas stream emanating from theprimary combustion chamber without increased concentrations ofpollutants being found in the flue gas from the afterburning chamber.However, the use of a pipe to connect the primary combustion chamberwith the afterburning chamber represents a drawback in anindustrial-scale installation (wear, caking).

SUMMARY OF THE INVENTION

The invention seeks to avoid these drawbacks. Accordingly, one object ofthe invention is to provide a novel process for the thermal treatment ofsolid materials, in particular refuse, in which the solid materials areburnt/gasified or pyrolized in a first step with a lack of oxygen, andthen the emerging gases are mixed with the oxygen-containing mediumwhich is required for complete burn-off and are burnt, in which processlocal concentration and temperature fluctuations in the flue gas fromthe first step are eliminated and as a result the pollutantconcentrations, in particular the NOx emissions, are minimized.

According to the invention, this is achieved by the fact that, for thepurpose of NOx reduction, the flue gases emerging from the first step,before they are mixed with the oxygen-containing medium in a mixingzone, are actively homogenized with the addition of a gaseous,oxygen-free or low-oxygen medium, and the homogenized, low-oxygenflue-gas stream emerging from the mixing zone, before theoxygen-containing medium which is required for complete burn-off isadded, passes through a holding zone, the residence time in the holdingzone being at least 0.5 second.

The advantages of the invention consist in the fact that the gasesemerging from the first step, due to their subsequent homogenization, nolonger exhibit any concentration and temperature fluctuations when theyare mixed with the burn-off air. The additional residence time for thehomogenized gas stream in the holding zone with a lack of air(substoichiometric air ratio) allows the NO which has already beenformed to be reduced by the NH_(x), HCN and CO present to form N₂.Consequently, only minimal pollutant emissions are formed in the thermaltreatment according to the invention of the solid materials.

It is particularly expedient if recirculated flue gas, water steam,oxygen-depleted air or inert gases, such as for example nitrogen, areused as gaseous oxygen-free or low-oxygen media for homogenization.These gases are advantageously injected into the mixing zoneperpendicular to the direction of flow of the flue gases or, in order toimprove the homogenization and mixing effect still further, are injectedat a certain angle and in the opposite or same direction to thedirection of flow of the flue gas from the first step.

Furthermore, it is advantageous if the active homogenization of the fluegases emerging from the first step is carried out with the aid ofcomponents (static mixing elements) which are installed in the mixingzone. These installed components divert the flow of the flue gases andconsequently cause them to be efficiently and intimately mixed. It isexpedient if these installed components have cavities through which acooling medium, e.g. water, water steam or air, flows.

Finally, it is advantageous for the active homogenization of the fluegases emerging from the first step to be carried out by means ofconstrictions or widenings of the cross section of the flow channel.

Moreover, it is expedient to control the temperature of the flue gasesin the area where the oxygen-containing medium is injected by means ofthe amount of oxygen-free or low-oxygen gaseous medium which is fed tothe mixing zone. This represents a very simple way of keeping thetemperature constant.

It is advantageous if a grate system with center-current firing or withcountercurrent firing is used as the first step.

Furthermore, it is advantageous if a fluidized bed is used as the firststep, since this provides a very good mass and heat transfer effect.Local temperature peaks and locally increased wear to the refractorylining can be prevented. Moreover, the ferrous and nonferrous metalscontained in the waste can be recovered from the ash with a very goodquality.

It is also expedient if the afterburning zone is a fluidized bed and theoxygen-containing gaseous medium is fed to the entry to the fluidizedbed or directly into the fluidized bed. It is then advantageouslypossible, due to the increased heat transfer caused by the presence ofparticles, to avoid local hot zones with a high level of thermal NOxformation. Moreover, caking on the heat-exchanger walls is prevented,with the result that the corrosion on the heat-exchanger surfaces isreduced. It is possible to set higher steam pressures and temperatures,allowing a higher thermal efficiency of the combustion installation tobe achieved.

Finally, it is expedient if the holding zone is a fluidized bed and thegaseous oxygen-free or low-oxygen medium is fed to the entry to thefluidized bed or directly into the fluidized bed.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are disclosed in the followingdescription and illustrated in the accompanying drawings, in which:

FIG. 1: shows a partial longitudinal section through an installation forthe thermal treatment of waste, in a first variant embodiment of theinvention in which a combustion grate is used an the first step;

FIG. 2: shows a partial longitudinal section through an installation forthe thermal treatment of waste in a second variant embodiment of theinvention in which a fluidized bed is used as the first step;

FIG. 3: shows a partial longitudinal section through an installation forthe thermal treatment of waste in a third variant embodiment of theinvention in which a combustion grate is used as the first step and afluidized bed is used as the afterburning zone;

FIG. 4: shows a partial longitudinal section through an installation forthe thermal treatment of waste in a fourth variant embodiment of theinvention in which a combustion grate is used as the first step and afluidized bed is used as the holding zone;

FIG. 5: shows a partial longitudinal section through an installationwhich is similar to that shown in FIG. 3 and in which a circulatingfluidized bed forms the afterburning zone.

Only those parts which are essential to gain an understanding of theinvention are shown. The direction of flow of the media is indicated byarrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, FIG. 1diagrammatically shows part of an installation for the thermal treatmentof solid materials, e.g. waste or coal, in a first variant embodiment ofthe invention. Waste is to be used in the present exemplary embodiment.

A grate 2 is arranged in the bottom part of a boiler 1, of which onlythe first flue is shown and the further radiation flues and theconvection part of which are not shown in FIG. 1. The waste-incinerationplant shown is designed with a center-current grate firing, i.e. theafterburning chamber 14 is arranged in the center above the grate 2.

The solid materials 3, in this case waste, are introduced into theboiler 1 and come to lie on the grate 2. Primary air 4 is blown in frombelow through the grate 2. Since only a small quantity of primary air 4is supplied, the lack of air or oxygen means that only a partialcombustion or a gasification of the waste takes place in this firstprocess step 5. CO-containing and low-O₂ flue gases 6 are formed in thisfirst step 5 and then flow into a mixing zone 7. The flue gas 6 emergingfrom the first step 5 is homogenized in this mixing zone 7.

In order to achieve homogenization, at least one virtually oxygen-freeor low-oxygen gaseous medium 8 is added in the mixing zone 7. In thepresent exemplary embodiment, on the one hand water steam 9 and on theother hand recirculated flue gas 10 are added as the medium 8. Nitrogenor other inert gases, and also air with a reduced oxygen content, arelikewise suitable for homogenization of the flue gas 6 from the firststep 5. In this case, it is sufficient if one of these media 8 isintroduced into the mixing zone 7, but mixtures of these different media8 are, of course, also suitable. As shown in FIG. 1, in this exemplaryembodiment the gaseous medium 8 is injected into the mixing zone 7approximately perpendicular to the direction of flow of the flue gases6.

Even more intensive mixing and homogenization is achieved if the medium8 is added at an angle in the opposite direction to the direction offlow of the flue gases 6 from the first process step 5. It is alsopossible to add the medium 8 at an angle in the same direction as thedirection of flow of the flue gases 6 from the first process step 5. Ahigh elevated pressure of the medium 8 also improves the homogenizationeffect.

In the present example, the mixing zone 7 is notable for variations inthe cross-sectional area of the walls of the boiler 1, i.e. forvariations 11 in the cross-sectional area of the flow channel. Thesevariations in cross section may be either constrictions or widenings ofthe flow channel. The variations 11 in cross section assist withhomogenization of the flue gases.

Furthermore, in the present exemplary embodiment in accordance with FIG.1, additional installed components 12 (static mixing elements) arearranged in the mixing zone 7, which components ensure that the flow ofthe flue gases 6 is diverted and therefore ensure further mixing andactive homogenization of the flue gases 6. The static mixing elements 12have cavities (not shown in the figure) through which coolant, e.g. air,water or water steam, flows.

Naturally, in other exemplary embodiments the various technical meansmentioned above (addition of a gaseous, virtually oxygen-free medium,installed components in the gas flow, variations in the cross-sectionalarea of the flow channel) may in each case be used as alternatives forhomogenization of the flue gases 6 from the first step 5.

The homogenized CO-rich flue gas emerging from the mixing zone 7 thenpasses into a holding zone 13, in which there is also a lack of oxygen,i.e. a substoichiometric air ratio in present. In the holding zone 13,some of the NO which has already been formed from the combustion isreduced in the presence of CO, NR₁ and HCN to form N₂. It is of primaryimportance for the invention that the residence time of the homogenizedflue gases in the holding zone 13 be at least 0.5 second. Given astandard flue-gas speed of approximately 4 m/s, this means that theholding zone must be at least approximately 2 m long.

Then, the flue gas flows out of the holding zone into the afterburningzone 14. There, an oxygen-containing medium 15, for example air(secondary air), is added, in order to ensure complete burn-off of theflue gas.

The novel process for the zoned thermal treatment of solid materials isdistinguished by simple process steps and by a reduced level of NOxemissions compared to the known prior art. In this case, in contrast tothe known prior art, the gas 6 emerging from the first step 5 is mixedand homogenized not in the afterburning zone by means of secondary air,but rather in an additional mixing zone 7 before the actualafterburning, a holding zone 13 for the flue gas, with a lack of oxygen,being incorporated between the mixing of the flue gases 6 and the supplyof the burn-off air 15, in which holding zone the gases have to stay forat least 0.5 second. In this way, it is possible both to reducepollutant emission levels and to achieve complete burn-off.

Furthermore, it is very simple, using the process according to theinvention, to control the temperature of the flue gases in the areawhere the oxygen-containing medium 15 is injected, by simply varying theamount of medium 8 fed into the mixing zone 7 and adapting theprevailing operating conditions.

FIG. 2 shows a further exemplary embodiment of the invention, whichdiffers from the first exemplary embodiment only in that a fluidized bed16 is used instead of the combustion grate in the first process step 5.The waste 3 is burnt under substoichiometric conditions in the fluidizedbed 16, advantageously resulting in a very good mass and heat transferand preventing local temperature peaks. As in the first exemplaryembodiment, the gas 6 emerging from the fluidized bed 16 (first step 5)is mixed and homogenized in the subsequent mixing zone 7, into which agaueous, virtually oxygen-free or low-oxygen medium 8, e.g. water steamn9 recirculated flue gas 10, is introduced and, moreover, in which staticinstalled components 12 are arranged which divert the flue gases 6 andtherefore bring about intensive mixing and homogenization. Thehomogenized Co-rich flue gas emerging from the mixing zone 7 then passesinto a holding zone 13, in which there is again a lack of oxygen. In theholding zone 13, some of the NO which has already been formed from thecombustion is reduced in the presence of CO, NH₁ and HCN to form N₂. Theflue gas then flows out of the holding zone 13 into the afterburningzone 14. There, an oxygen-containing medium 15, for example air, isadded, in order to ensure complete burn-off of the flue gas.

FIG. 3 shows an exemplary embodiment in which, in contrast to theexample illustrated in FIG. 1, the afterburning zone 14 is designed as afluidized bed 16. The oxygen-containing gaseous medium 15 is eitherintroduced directly into the fluidized bed 16 or is introduced at theentry to the fluidized bed 16. Both these alternatives are illustratedin FIG. 3. By designing the afterburning zone 14 as a fluidized bed 16,it is possible, due to the high level of heat transfer caused by thepresence of particles, to avoid local hot zones with high levels ofthermal NOx formation. Moreover, it is possible to prevent caking onheat-exchanger walls and to considerably reduce the corrosion atheat-exchanger surfaces. It Is also possible to set higher steampressures and temperatures, allowing higher thermal efficiency of thecombustion installation to be achieved.

FIG. 4 shows a partial longitudinal section through an installation forthe thermal treatment of waste in a fourth variant embodiment of theinvention, in which a combustion grate 2 is used as the first step and afluidized bad 16 is used as the holding zone 13. In contrast to FIG. 1,in this exemplary embodiment the mixing zone 7 is characterized by awidening in the cross section. Then, with the homogenized flue gasemerging from the mixing zone 7, intensive mass and heat transferadvantageously take place in the fluidized bed 16 (holding zone 13).

Finally, FIG. 5 shows a further variant embodiment, which differs fromthat shown in FIG. 3 only in that the fluidized bed 16 in theafterburning zone 14 is in this case a circulating fluidized bed, inwhich the empty pipe velocity in the riser is increased. The fluidizedmaterial is discharged into a cyclone and to then returned to thefluidized bed. The average vertical gas velocity in the riser is higherin the circulating fluidized bed than in the conventional fluidized bed,and the average relative velocity between gas and particles alsoincreases. This leads to an increased heat and mass transfer between gasand particles and therefore to a reduced temperature and concentrationdistribution. In addition, by using an external fluidized-bed cooler, itis possible to vary the amount of heat withdrawn from the fluidized bedand thus to correctly set the fluidized-bed temperature and thetemperature at the end of the afterburning zone.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein. For example, in another exemplary embodiment, the holding zone13 may also be designed as a circulating fluidized bed, or alternativelya grate system with countercurrent firing may be used.

While the present invention has been described by reference to theabove-mentioned embodiments, certain modifications and variations willbe evident to those of ordinary skill in the art. Therefore the presentinvention is to be limited only by the scope and spirit of the appendedclaims.

LIST OF DESIGNATIONS

1 Boiler

2 Grate

3 Solid material, for example waste

4 Primary air

5 First process step

6 Flue gas from pos. 5

7 Mixing zone

8 oxygen-free or low-oxygen gaseous medium

9 Water steam

10 Recirculated flue gas

11 Variations of cross-sectional area of the flow channel

12 Installed components/static mixing elements

13 Holding zone

14 Afterburning zone

15 Oxygen-containing gaseous medium

16 Fluidized bed

What is claimed:
 1. A process for the thermal treatment of solidmaterials in which the solid materials are burnt/gasified or pyrolizedin a first step with a lack of oxygen, and then, in an afterburning zoneflue gases from the first step are mixed with an oxygen-containinggaseous medium and are burnt with complete burn-off, wherein, the fluegases emerging from the first step, before the flue gases are mixed withthe oxygen-containing medium in a mixing zone, are actively homogenizedwith the addition of a gaseous, oxygen-free or low-oxygen medium intothe mixing zone, and the homogenized, low-oxygen flue-gas streamemerging from the mixing zone, before the oxygen-containing medium whichis required for complete burn-off is added, passes through a holdingzone, the residence time in the holding zone being at least 0.5 second.2. The process as claimed in claim 1, wherein the oxygen free or lowoxygen used is recirculated flue gas.
 3. The process as claimed in claim1, wherein the oxygen free or low oxygen medium used is water steam. 4.The process as claimed in claim 1, wherein the oxygen free or low oxygenmedium used is oxygen-depleted air.
 5. The process as claimed in claim1, wherein the oxygen free or low oxygen medium used is inert gas. 6.The process as claimed in claim 1, wherein the active homogenization ofthe flue gases emerging from the first step is carried out with the aidof components which are installed in the mixing zone.
 7. The process asclaimed in claim 6, wherein a cooling medium of water, steam or air,flows through the installed components.
 8. The process as claimed inclaim 1, wherein the active homogenization of the flue gases emergingfrom the first step is carried out by means of constriction or wideningsof a cross section of a flow channel in the mixing zone.
 9. The processas claimed in claim 1, wherein the temperature of the flue gases in anarea where the oxygen-containing medium is injected is controlled by theamount of oxygen containing medium supplied to the mixing zone.
 10. Theprocess as claimed in claim 1, wherein in the holding zone the fluegases have a substoichiometric air ratio.
 11. The process as claimed inclaim 1, wherein a grate system with center-current grate firing is usedin the first step.
 12. The process as claimed in claim 1, wherein agrate system with countercurrent grate firing is used in the first step.13. The process as claimed in claim 1, wherein a fluidized bed is usedin the first step.
 14. The process as claimed in claim 1, wherein theafterburning zone is a fluidized bed, and wherein the oxygen-containinggaseous medium is fed either to the flue gas when the oxygen containinggaseous medium enters the fluidized bed or directly into the fluidizedbed.
 15. The process as claimed in claim 1, wherein the holding zoneincludes a fluidized bed, and wherein the oxygen-free or low-oxygengaseous medium is fed either to the flue gas when the oxygen free or lowoxygen gaseous medium enters the fluidized bed or directly into thefluidized bed.
 16. The process as claimed in claim 14, wherein thefluidized bed used is a circulating fluidized bed.
 17. A method ofthermally treating refuse comprising the steps of: i) partiallycombusting the refuse with a lack of oxygen thereby producing CO, NO andlow O₂-containing flue gasses; ii) passing the flue gasses into a mixingzone and homogenizing the flue gasses by introducing at least oneoxygen-free or low-oxygen medium into the mixing zone; iii) passing thehomogenized flue gasses into a holding zone comprising an oxygen-free orlow-oxygen environment and reducing at least a portion of the NOcontained in the homogenized flue gasses thereby forming N₂, wherein thehomogenized flue gasses reside in the holding zone for at least 0.5seconds; iv) passing the reduced homogenized flue gasses into anafterburning zone; and v) adding an oxygen-containing medium in theafterburning zone thereby promoting complete burnoff of the reducedhomogenized flue gasses.